The Collaborative International Dictionary of English v.0.48:

Christ's-thorn \Christ's-thorn`\, n. (Bot.)
   One of several prickly or thorny shrubs found in Palestine,
   especially the Paliurus aculeatus, Zizyphus
   Spina-Christi, and Zizyphus vulgaris. The last bears the
   fruit called jujube, and may be considered to have been the
   most readily obtainable for the Crown of Thorns.
   [1913 Webster]